A specific attention has been paid to a magnetic random access memory (to be referred to as “MRAM” hereinafter) as a non-volatile memory capable of a writing operation in a high speed, and also having a large number of times of rewrite.
As shown in FIG. 1, a memory cell of a typical MRAM contains a magnetic resistance element 104 composed of a pinned layer 101 having a fixed spontaneous magnetization, a free layer 102 having a spontaneous magnetization which can be reversed, and a non-magnetic material spacer layer 102 which is interposed between the pinned layer 101 and the free layer 102. The free layer 102 is formed in such a manner that the direction of the spontaneous magnetization of the free layer 102 can be set in parallel to or in anti-parallel to that of the spontaneous magnetization of the pinned layer 101.
The memory cell stores 1-bit data as the direction of the spontaneous magnetization of the free layer 102. The memory cell can take two states, namely a “parallel” state and an “anti-parallel” state. In the “parallel” state, the direction of the spontaneous magnetization of the free layer 102 and that of the spontaneous magnetization of the pinned layer 101 are parallel to each other. In the “anti-parallel” state, the direction of the spontaneous magnetization of the free layer 102 and that of the spontaneous magnetization of the pinned layer 101 are anti-parallel to each other. The memory cell stores the 1-bit data by setting any one of the “parallel” state and the “anti-parallel” state to “0”, and by setting the other to “1.”
A reading operation of data from the memory cell is carried out by sensing a change in resistances of the memory cell due to a magneto-resistance effect. The directions of the spontaneous magnetizations of the pinned layer 101 and the free layer 102 give an influence to resistances of the memory cell. When both the direction of the spontaneous magnetization of the pinned layer 101 and the direction of the spontaneous magnetization of the free layer 102 are parallel to each other, the resistance of the memory cell is a first value “R”. When both the direction of the spontaneous magnetization of the pinned layer 101 and the direction of the spontaneous magnetization of the free layer 102 are anti-parallel to each other, the resistance of the memory cell is a second value “R+ΔR”. Thus, the directions of the spontaneous magnetizations of the pinned layer 101 and of the free layer 102, namely, the data which has been stored in the memory cell can be discriminated by sensing the resistances of the memory cell.
A writing operation of data into the memory cell is carried out by supplying currents to signal lines (typically, a word line and a bit line) which are arranged in a memory cell array, and by reversing the direction of the spontaneous magnetization of the free layer 102 through application of a magnetic field which is produced by the currents.
A ferromagnetic layer of the free layer 102 has a magnetic field-magnetization characteristic (H-M characteristic) symmetrical with respect to the applied magnetic field essentially (namely, as nature of a bulk of the ferromagnetic). Therefore, a resistance value of the memory cell ideally represents a symmetrical characteristic with respect to a write magnetic field which is applied to the memory cell for data write, as shown in FIG. 2A.
However, as shown in FIG. 2B, an actual resistance value of the memory cell represents an asymmetrical characteristic with respect to the write magnetic field. That is to say, the characteristic has an offset magnetic field “Hoff”. Presence of the offset magnetic field “Hoff” implies that the write magnetic fields used to write data of “1” and “0” are different from each other. The offset magnetic field “Hoff” is not preferable since a memory operation margin is reduced, and a current required for writing data is increased.
As a cause of this offset magnetic field, the Orange Peel Coupling effect (or Neel's coupling effect) is known. J. C. S. Kools et al. report the generation of the offset magnetic field due to the Orange Peel Coupling effect in “Effect of finite magnetic film thickness on Neel's Coupling in spin valves” (Journal of Applied Physics, Vol. 85, p. 4466 (1999)).
As shown in FIG. 3, the orange peel coupling effect is caused by a non-flatness of the surfaces of the pinned layer 101 and the free layer 102. In the manufacturing steps of the memory cell of the MRAM, it is practically difficult to make the surfaces of the pinned layer 101 and the free layers 102 completely flat. In an actual case, the surfaces of the pinned layer 101 and the free layer 102 are waved. Waving of both the pinned layer 101 and the free layer 102 produces an interlayer coupling field between the spontaneous magnetizations of the pinned layer 101 and the free layer 102. This interlayer coupling field causes the offset magnetic field in the memory cell.
As another cause of the generation of the offset magnetic field, the Magneto-Static Coupling effect is known. This magneto-static coupling effect is also called as a “fringe effect”. As shown in FIG. 4, the magneto static coupling effect is caused since magnetic poles are generated at ends of the pinned layer 101. A magnetic field “HMS” which is generated by the magnetic poles is applied to the free layer 102. As a consequence, an asymmetrical characteristic is led in a reversion magnetic field by which the direction of spontaneous magnetization of the free layer 102 is reversed. This asymmetrical characteristic of the reversion magnetic field becomes the cause that the memory cell has the offset magnetic field.
U.S. Pat. No. 6,233,172 discloses the technique for eliminating the offset magnetic field in the memory cell by utilizing that the direction of the magnetic field generated based on the orange peel coupling effect and the direction of the magnetic field generated based on the magneto-static coupling effect are opposite to each other so that the orange peel coupling effect and the magneto-static coupling effect are cancelled.
FIG. 5 indicates the structure of the memory cell of the MRAM disclosed in the above-described U.S. patent. This structure is composed of a substrate 112, a lower electrode stacked layer film 114, a spacer layer 116, and an upper electrode stacked layer film 118. The lower electrode stacked layer film 114 is composed of a bottom layer 120, an anti-ferromagnetic layer 122, a pinned ferromagnetic layer 124, a ruthenium layer 126 and a fixed ferromagnetic layer 128. The upper electrode stacked layer film 118 is composed of a free ferromagnetic layer 130 and a protection layer 132. Both the pinned ferromagnetic layer 124 and the fixed ferromagnetic layer 128 have the spontaneous magnetizations whose directions are fixed, whereas the free ferromagnetic layer 130 has the spontaneous magnetization whose direction can be reversed.
The above-described US patent discloses that cancellation of the orange peel coupling effect and the magneto-static coupling effect can be achieved by satisfying the following equation:M1T1<M2T2.In this equation, a symbol “T1” indicates a film thickness of the pinned ferromagnetic layer 124, and a symbol “M1” indicates a magnetization of the pinned ferromagnetic layer 124. Also, a symbol “T2” indicates a film thickness of the fixed ferromagnetic layer 128, and a symbol “M2” indicates a magnetization of the fixed ferromagnetic layer 128. The above-described equation can be satisfied under the following conditions:T1<T2 and M1=M2,T1=T2 and M1<M2,or T1<T2 and M1<M2.
However, as shown in FIG. 6A and FIG. 6B, the magneto-static coupling effect is not uniform within the free layer 102. FIG. 6B shows a calculation result of the magnetic field applied to the free layer 102 due to the magneto-static coupling effect in a rectangular memory cell having the shape shown in FIG. 6A. The magnetic field received by the free layer 102 is large near the ends of the pinned layer 101, and is small on a position separated far from the ends of the pinned layer 101. As described above, the magnetic field received by the free layer 102 is not uniform, and it is difficult that the orange peel coupling effect is canceled by the magneto-static coupling effect over the entire inner portion of the memory cell.
Furthermore, recent progress in very fine technique for the MRAM may makes it difficult to cancel the orange peel coupling effect by the magneto-static coupling effect in each of large numbers of memory cells contained in the MRAM. The free layer is made thinner in accompaniment with the finer memory cell in the MRAM, so that the orange peel coupling effect influences strongly. In addition, dimensions of pinned layers are decreased, so that the magneto-static coupling effect influences strongly. The deviations of the magnitude of the orange peel coupling effect and that of the magneto-static coupling effect dependent on the manufacturing process increases. As a consequence, the deviations of these effects in every memory cell increase. The increase of these deviations may makes it difficult to cancel the orange peel coupling effect and the magneto-static coupling effect in each of the memory cells, and disturbs that the offset magnetic field is eliminated in each of these memory cells contained in the memory cell arrays of the MRAM.
Japanese Laid-Open Patent Applications (JP-P2000-332318 and JP-A-Heisei 10-188235) disclose a structure of a magneto-resistance element used to provide a highly reliable magnetic head having a high sensitivity by controlling an external magnetic field applied to a free layer. However, these conventional examples do not describe that an offset magnetic field of the free layer of the magneto-resistance element is eliminated. The magnetic head requires that the free layer is located at a proper bias point, but do not require that the offset magnetic field of the free layer is eliminated.
Under such a circumstance, a technique for effectively reducing an offset magnetic field in a memory cell is demanded. In particular, the techniques are demanded in which the offset magnetic field can be uniformly reduced over the whole of internal portion in the memory cell, and the offset magnetic field can be eliminated in each of the memory cell without being adversely influenced by a manufacturing deviation.